PHYSIOLOGICAL RESPONSES OF GINKGO BILOBA SEEDLINGS TO CRETACEOUS-LIKE ATMOSPHERIC COMPOSITIONS

Research has demonstrated the presence of morphological variability in Cretaceous plants, but few studies have investigated ecophysiological parameters.  The Cretaceous atmospheric environment (e.g., 30% O₂, 2000 ppm CO₂, 1.25 atm pressure) was significantly different from the modern atmosphere (21% O₂, 370 ppm CO₂, 1.0 atm pressure); hence unique gas exchange and physiological responses are likely. Ginkgo biloba seedlings were placed in hyperbaric chambers filled with the following gas mixes.  Relevant atmospheric parameters were systematically varied between mixes as a means of approximating Cretaceous environments:

1. Control: ambient conditions, no alteration

2. Elevated pressure to 1.25 atm, ambient CO₂ and O₂

3. 1.25 atm pressure, CO₂ enriched to 2000 ppm, ambient O₂

4. 1.25 atm pressure, O₂ concentration enriched to 30%, ambient CO₂

5. 1.25 atm pressure, CO₂ enriched to 2000 ppm and O₂ enriched to 30%

Two experiments were completed. The first tested instantaneous net photosynthetic rate responses to a range of CO₂ concentrations after 24 hours of acclimation. Results show an increase in photosynthetic rate of 200-300% with highest CO₂. In the second longer-term experiment, plants were exposed for five weeks to allow leaves to develop.  At the end of this period, gas exchange, stomatal density and leaf chemical constituents were measured. Results from both experiments indicate a response towards higher rates of primary productivity with elevated CO₂, irrespective of other atmospheric alterations. Hence plants seem more sensitive to changes in CO₂ than to changes in O₂ or pressure. The ramifications are that paleo-ecophysiological response modeling can be simplified to a question of CO₂ concentration only; O₂ concentration is less relevant. These studies provide valuable insights into the ecophysiology of terrestrial ecosystems during the Cretaceous.